Phytosterolaemia (sitosterolaemia) is a very rare inherited sterol storage disease characterized by tendon and tuberous xanthomas and by a predisposition to atherosclerosis. We here describe the first Scandinavian case. The 14-year-old female patient was found to have markedly elevated circulating levels of plant sterols (sitosterol, sitostanol, campesterol, stigmasterol), and the levels of these sterols were 20-50 times higher than in her healthy sister and heterozygous parents. In addition to the usual serum plant sterols we found a new major sterol in the patient tentatively identified as episterol or fecosterol (24-methyliden-cholest-7 (or 8)-en-3 beta-ol). A newly developed method based on the use of deuterium labelled cholesterol and plant sterols was used to measure sterol absorption in the patient and her relatives. Absorption of sitosterol averaged 20% in the patient and ranged from 4 to 8% in the relatives. Absorption of campesterol averaged 31% in the patient and ranged from 15 to 18% in her relatives. Absorption of cholesterol averaged 63% in the patient and ranged from 35 to 45% in the relatives. Cholesterol synthesis appeared to be reduced in the patient and was 46-52% of that of her relatives.